in certain Patterns of Blue and Red Worsted. 347 



side, we are led to infer that, as the lamp or candle is station- 

 ary, the spots themselves actually shift their position, and 

 move faster than the ground of the pattern. Such at least is 

 the illusion ; and in some instances the spots will even appear 

 like protuberant bodies rolling from side to side. 



In order to prove that this change of tint at the edges of 

 the figures is an efficient cause of the illusion, I prepared a 

 small instrument on this principle. A blue and a red spot 

 were affixed to a white card in such a manner that moveable 

 shadows might be introduced behind them, which were made 

 to slide from side to side in imitation of the borders of the 

 worsted spots, and to persons not acquainted with the nature 

 of the contrivance the effect was the same, the coloured spots 

 appeared to move when their shadows only were being moved. 



On the other hand, when the formation of the light and 

 dark borders is prevented, as may be done by inserting a row 

 of white stitches round each of the spots, so as to separate the 

 two colours, then the illusion ceases — the figures are no longer 

 seen to move. The reason of this is obvious, that the pre- 

 sence of the white stitches is an obstacle to the blending of 

 the colours on which the light and dark borders depend for 

 their production. The difference of tint in the two cases may 

 require some further explanation, — why the red spot has a 

 dark border, and the blue a light one. The apparent blend- 

 ing of colours in a state of rapid motion is generally accounted 

 for by the physiological fact, that the eye retains impressions 

 for an appreciable time after the objects themselves are with- 

 drawn from view ; and when a number of objects are presented 

 to the eye in quick succession, the image left on the retina by 

 one object is liable to be confused by that of the succeeding 

 object; and when these are of different colours, a compound 

 colour is perceived which is intermediate between the two ; 

 but as the spectrum left by the first object is fainter than the 

 image of that which is actually present, it follows that the 

 colour of the latter will always predominate. Hence in the 

 blending of the two colours which takes place at the junction 

 of the spot and the ground of the pattern, the resulting tints 

 are different in each case ; for the red spot it is purple, 

 that is, the margin of the red spot is seen through a blue 

 spectrum, that of the ground, — while the border of the blue 

 spot appears of a lavender tint, the blue being seen through 

 the spectrum of the red ground. From experiments made 

 with glass of different colours, I venture to suggest this as the 

 probable explanation ; for I find that red becomes darker by 

 being seen through a medium of light blue, while the blue is 

 rendered lighter by the aid of a red glass. 



2 A 2 



